This invention pertains to a precision bearing structure wherein a ball bearing having a plurality of balls and two races supports a rotatable shaft on the inner race and a housing on its outer race.
Precision ball bearings, for example on the gimbals of stabilized platforms used in guidance systems, may be attached by adhesive to the surrounding housing and to the shaft. Unfortunately, fastening the races with adhesive prevents the removal of bearings.
If the bearing races are of a different material than the housing and shaft, the races have a different coefficient of thermal expansion than the housing and shaft.
For example, it may be desirable to fabricate the housing and shaft of aluminum and to fabricate the bearing races of steel. The aluminum has a higher thermal coefficient of expansion than the steel. When the temperature is decreased, the housing and shaft pulls away from the bearing races. When the temperature is increased, the housing and shaft tightens onto the steel bearing races and stresses the aluminum housing and shaft.
To avoid problems of differential thermal expansion while still attaching the steel races to the aluminum housing and shaft, radial cuts have been made through the races, and a small segment of the races is removed. The bearing races are axially loaded so that the steel races continually load the balls and are pushed into solid contact with both the aluminum housing and shaft. Differential thermal expansion between the aluminum and steel merely opens and closes the gap in the races and varies the load angle on the balls. The races are maintained at a tight fit with both the shaft and housing. However, because of the space left by the segment removal, the bearings are easily inserted into and removed from the housing, and the shaft is easily inserted into and removed from the inner race. Unfortunately, when the balls cross the gap left by the removed segments in the races, a short-duration torque is observed between the races.
To avoid the torque pulses between the races, ramps may be machined onto the ball track where the balls contact the gap. Although such ramps remove the torque pulses, when the gaps in both of the races are radially aligned, because of the large spacing between the races in the region of the gaps, the balls are unloaded. Further, such ramps are very difficult to machine.